In conventional double inlet centrifugal fans such as shown in Patent Document 1 below, the fan casing is provided with a motor-side casing inlet port and an opposite-motor-side casing inlet port having a larger inner diameter than the motor-side casing inlet port.
The double inlet centrifugal fan of Patent Document 1 will be described as follows with reference to FIG. 13 which is a side view where a unit including the fan is installed and FIG. 14 which is a plan view of the unit.
As shown in FIGS. 13 and 14, box-shaped unit 106 is installed on attic 101 floor in such a manner as to be connected to suction-side duct 103 at unit air inlet 102 and to discharge-side duct 105 at unit air outlet 104. Unit air inlet 102 is provided on one side of unit 106 to draw in outdoor air, and unit air outlet 104 is provided on the other side thereof to supply the outdoor air into a room. Unit 106 includes double inlet centrifugal fan 112 and heat exchanger 113. Double inlet centrifugal fan 112 includes fan casing 107 and motor 108. Fan casing 107 includes opposite-motor-side casing inlet port 109 and motor-side casing inlet port 110. Motor 108 has an outer diameter 111 substantially equal to the diameter of opposite-motor-side casing inlet port 109. Motor-side casing inlet port 110 has a larger diameter than opposite-motor-side casing inlet port 109.
When double inlet centrifugal fan 112 is operated, outdoor 114 air is drawn into heat exchanger 113 through suction-side duct 103. After discharged from heat exchanger 113, some of the air passes through motor-side air passage 115 and is drawn through motor-side casing inlet port 110, whereas the other passes through opposite-motor-side air passage 116 and is drawn through opposite-motor-side casing inlet port 109. These air flows pass through discharge-side duct 105 via double inlet centrifugal fan 112 and are supplied indoors 120. Double inlet centrifugal fan 112 includes impeller 119 having disk-shaped main plate 118 and a plurality of blades on both sides of main plate 118. The blades have inner diameter 117 substantially equal to the diameter of motor-side casing inlet port 110.
As shown in Patent Document 2 below, some impellers that can be used in double inlet centrifugal fans of this type have blades whose main plate-side outlet angle is made smaller than their fan-side-plate-side outlet angle.
An impeller that can be used in double inlet centrifugal fans of this type is described as follows with reference to FIGS. 15 to 17A and 17B. FIGS. 15 and 16 are a plan view and a side view, respectively, of a conventional double inlet centrifugal fan. FIGS. 17A and 17B are enlarged side views of the fan; the former showing relative speed W1 of the main plate-side fluid and the latter showing relative speed W2 of the fan-side-plate-side fluid.
In double inlet centrifugal fan 132 shown in FIGS. 15 and 16, single inlet fan casing 126 includes outlet port 121, spiral scroll 122, casing side plates 123 on both sides of fan casing 126, and orifice 125 having casing inlet port 124 on one of casing side plates 123. Fan 132 further includes single inlet impeller 131 having disk-shaped main plate 128, annular fan side plates 129, and a plurality of blades 130 disposed between main plate 128 and each of side plates 129. Main plate 128 is connected to drive shaft 127 of fan casing 126.
Blades 130 have main-plate-side outlet angle 133 of α1 and fan-side-plate-side outlet angle 134 of α2 in the relation that α1<α2. When fan 132 is operated, the air drawn through casing inlet port 124 is discharged through outlet port 121 showing wind speed distribution 135 along blades 130.
As shown in the speed triangles of FIGS. 17A and 17B, when relative speed 136 of the main plate-side fluid is W1, relative speed 137 of the fan-side-plate-side fluid is W2, and circumferential speed 138 is U, W1 tends to be larger than W2 as shown in wind speed distribution 135. When circumferential component 139 of the absolute velocity of the main plate-side discharge current is CU1, and circumferential component 140 of the absolute velocity of the fan-side-plate-side discharge current is CU2, CU2 approaches CU1 because of the relation that α1<α2. This equalizes the total pressure increase in blades 130 along drive shaft 127.
When such a conventionally-shaped impeller is used in conventional fan 132, air flow is decreased due to the air passage resistance in the unit. Therefore, fan 132 uses a large-diameter motor to increase the revolution, thereby ensuring sufficient air flow. However, the motor with a larger outer diameter blocks the motor-side casing inlet port, requiring an increase in the diameter of the motor-side casing inlet port. As a result, the air passage area obtained by subtracting the area of the motor from the area of the motor-side casing inlet port is made equal to the area of the opposite-motor-side casing inlet port.
In this impeller, the blades on the motor side and the blades on the opposite motor side have a substantially equal inner diameter. And the casing inlet port on the motor side has a large diameter substantially equal to the inner diameter of the blades. The opposite-motor-side casing inlet port has air inlet resistance because it has a diameter smaller than the inner diameter of the blades. The resistance is reduced by using a large-diameter impeller so as to ensure sufficient air flow to the opposite-motor-side air passage. However, this results in an increases in the height of the fan casing and hence the height of the unit.
When the impeller that can be used in conventional double inlet centrifugal fan 132 is used in a single inlet fan, the current is guided to blades 130 through a single inlet port. As a result, the relative speeds of the fluids over the blades have a small difference along the drive shaft, so that the total pressure increase in the blades can be equalized along the drive shaft by adjusting the outlet angle.
When the impeller is used in a double inlet centrifugal fan, on the other hand, the current is guided to the blades on both sides of the main plate through two inlet ports. In this case, the motor arrangement on one side or the entire arrangement in the unit causes air passage resistance. This makes the relative speeds of the fluids over the blades largely different between both sides of the main plate. Consequently, the adjustment of the outlet angles cannot be fully accomplished only by changing them, and therefore it is difficult to equalize the total pressure increase in the blades along the drive shaft. Sufficient air flow is ensured by using a large-diameter impeller; however, this results in an increase in the height of the fan casing and hence the height of the unit.    Patent Document 1: Japanese Patent Unexamined Publication No. H03-175199    Patent Document 2: Japanese Patent Unexamined Publication No. H09-195988